Field
Embodiments disclosed herein generally relate to data storage and computer memory systems, and more particularly, to an MRAM device with improved spin torque efficiency.
Description of the Related Art
The heart of a computer is a magnetic recording device which typically may include a rotating magnetic media or a solid state media device. A number of different memory technologies exist today for storing information for use in a computing system. These different memory technologies may, in general, be split into two major categories: volatile memory and non-volatile memory. Volatile memory may generally refer to types of computer memory that requires power to retain stored data. Non-volatile memory, on the other hand, may generally refer to types of computer memory that do not require power in order to retain stored data. Examples of volatile memory may include certain types of random access memory (RAM), such as dynamic RAM (DRAM) and static RAM (SRAM). Examples of non-volatile memory may include read-only memory (ROM), magnetoresistive RAM (MRAM), and flash memory, such as NOR and NAND flash, etc.
In recent years there has been a demand for higher density devices, which maintain a relatively low cost per bit, for use in high capacity storage and memory applications. Today the memory technologies that generally dominate the computing industry are DRAM and NAND flash; however these memory technologies may not be able to address the current and future capacity demands of next generation computing systems.
Recently, a number of emerging technologies have drawn increasing attention as potential contenders for next generation memory. One such memory technology is magnetoresistive random access memory (MRAM). MRAM offers fast access time, infinite read/write endurance, radiation hardness, and high storage density. Unlike conventional RAM chip technologies, MRAM data is not stored as an electric charge, but instead stores data bits using magnetic charges. The elements are formed from two magnetically polarized layers, each of which can maintain a magnetic polarization field, separated by a thin insulating layer, which together form a magnetic tunnel junction (MTJ) structure. MRAM cells including MTJ memory elements can be designed for in-plane or perpendicular magnetization of the MTJ layer structure with respect to the film surface. One of the two layers (referred to as a fixed or reference layer) has its magnetization fixed and set to a particular polarity, for example by coupling the layer to an antiferromagnet; the polarization of the second layer (referred to as a free layer) is free to rotate under the influence of an external writing mechanism such as a strong magnetic field or a spin polarized electric current (which is used in a form of MRAM know as spin-torque transfer or STT-MRAM). Therefore, the cells are typically designed to have two stable states (i.e. a “0” or a “1” defined by the resistance of the MTJ) that allow the cells to serve as non-volatile memory cells.
Additionally, a hindrance to the scaling of STT-MRAM densities to values approaching DRAM is the amount of current required for switching individual bits, as the current available for switching is limited by complementary metal oxide semiconductor (CMOS) sense transistors with values on the order of tens of microamps at 20 nm node sizes. Due to these limitations, it is critical to reduce the amount of current required to switch the magnetization state of the bits without affecting the switching reliability, endurance, and signal to noise (SNR) of the bit. Many strategies have been introduced to reduce switching currents, such as moving from layers polarized in the film plane to layers polarized perpendicular to the film plane, as this reduces the torque required to switch the free layer. However, this method alone is not sufficient to lower the switching currents to the values mentioned above. Additional methods for reducing switching currents have been proposed such as inducing weak perpendicular magnetic anisotropy (PMA) on the switching (free) layer to tilt it slightly from its equilibrium position, or utilizing a tilted polarizing reference layer in a MRAM bit. Although modeling results have shown these concepts can help to reduce switching currents, there is a need for a physical system that incorporates the property of a tilted magnetic layer. Therefore, there is a need in the art for an improved STT-MRAM device which utilizes a reduced switching current.